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United States Patent |
5,216,347
|
Pace
,   et al.
|
June 1, 1993
|
Stepper motor control system
Abstract
A control system for controlling a stepper motor which uses six power
transistors in a single, interconnected configuration with two stepper
motor field coils. Two transistors are common to both field coils of the
stepper motor with two stepper motor field coils. Control circuitry for
switching the six transistors allows full and half step motor positioning
to be performed. Transition phases are used between steady state phases to
provide accurate motor operation. The transition phases are controlled
entirely within the control circuitry without a requirement for input from
outside the controller.
Inventors:
|
Pace; Ermanno (Phoenix, AZ);
McCormack; Mark J. (Phoenix, AZ)
|
Assignee:
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SGS-Thomson Microelectronics, Inc. (Carrollton, TX)
|
Appl. No.:
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677438 |
Filed:
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March 29, 1991 |
Current U.S. Class: |
318/696; 318/685 |
Intern'l Class: |
H02P 008/00 |
Field of Search: |
318/685,696
|
References Cited
U.S. Patent Documents
4107593 | Aug., 1978 | Anderson | 318/685.
|
4558268 | Dec., 1985 | Besson et al. | 318/685.
|
4908562 | Mar., 1990 | Back | 318/685.
|
Primary Examiner: Shoop, Jr.; William M.
Assistant Examiner: Masih; Karen
Attorney, Agent or Firm: Hill; Kenneth C., Jorgenson; Lisa K., Robinson; Richard K.
Claims
What is claimed is:
1. A controller for a stepper motor having first and second coils,
comprising:
a common electrical node connecting a first end of each coil;
a first transistor switch connected between said common node and a first
power supply node;
a second transistor switch connected between said common node and a second
electrical node;
third and fourth transistor switches connected between the first power
supply node and second ends of the first and second coils, respectively;
fifth and sixth transistor switches connected between the second electrical
node and the second ends of the first and second coils, respectively;
control circuitry including a state machine defining a plurality of normal
states and a plurality of transition states, each of said normal and
transition states corresponding to a selected pattern of on and off states
for said transistor switches; and
decay current sense circuitry for sensing current flow through said coils
during a transition state;
wherein the rotor is moved from a first to a second position in response to
a control input signal, each position having a corresponding normal state,
and wherein said control circuitry causes said state machine to move
through a selected transition state between the normal states, with said
state machine remaining in such transition state until said decay current
sense circuitry senses that current flow through said coils has reached a
predetermined value, at which time said state machine moves into the
normal state corresponding to the second rotor position.
2. The controller of claim 1, wherein said transistor switches comprise
power field effect transistors.
3. The controller of claim 1, wherein the transition states are defined so
that an exponential decay current through one coil decays relatively
slowly, while a current through the other coil increases or decreases at a
relatively fast rate.
4. A stepper motor positioning system, comprising:
a stepper motor having a rotor and a stator;
first and second field coils wound adjacent to said stator;
first, second, and third transistor switch pairs, each pair having two
transistor switches connected in series, with a first, second, and third
electrical node respectively connected between the transistor switches of
each switch pair;
wherein said first coil is connected between the first and second nodes,
and said second coil is connected between the second and third nodes;
control circuitry including a state machine defining a plurality of normal
states and a plurality of transition states, each of said normal and
transition states corresponding to a selected pattern of on and off states
for said transistor switches; and
decay current sense circuitry for sensing current flow through said coils
during a transition state;
wherein the rotor is moved from a first to a second position in response to
a control input signal, each position having a corresponding normal state,
and wherein said control circuitry causes said state machine to move
through a selected transition state between the normal states, with said
state machine remaining in such transition state until said decay current
sense circuitry senses that current flow through said coils has reached a
predetermined value, at which time said state machine moves into the
normal state corresponding to the second rotor position.
5. The positioning system of claim 4, wherein said transistor switches
comprise power field effect transistors.
6. The positioning system of claim 4, wherein said control circuitry
includes:
a detector for detecting a level of current flowing through said coils; and
a current controller for limiting the current flowing through said coils to
a preselected value.
7. The positioning system of claim 6, wherein said current controller
rapidly switches selected ones of said transistor switches on and off in
response to the level of current flowing through said coils.
8. The positioning system of claim 4, wherein said decay current sense
circuitry comprises:
a sense resistor in series with current flowing through said coils; and
a voltage comparator connected to said sense resistor for comparing a
voltage drop across said resistor with preselected voltage values.
9. A stepper motor positioning system, comprising:
a stepper motor having a rotor and a stator;
first and second filed coils wound adjacent to said stator;
first, second, and third transistor switch pairs, each pair having two
transistor switches connected in series, with a first, second, and third
electrical node respectively connected between the transistor switches of
each switch pair;
wherein said first coil is connected between the first and second nodes,
and said second coil is connected between the second and third nodes;
control circuitry including a state machine defining a plurality of normal
states and a plurality of transition states, each of said normal and
transition states corresponding to a selected pattern of on and off states
for said transistor switches;
a sense resistor in series with current flowing through said coils;
a voltage comparator connected to said sense resistor for comparing a
voltage drop across said resistor with preselected voltage values; and
first and second sense transistor switches connected respectively to the
first and third nodes, wherein said sense transistors switch state when a
voltage at the corresponding node rises above and falls below a
preselected value;
wherein the rotor is moved from a first to a second position in response to
a control input signal, each position having a corresponding normal state,
and wherein said control circuitry causes said state machine to move
through a selected transition state between the normal states, with said
state machine remaining in such transition state until said voltage
comparator senses that current flow through said coils has reached a
predetermined value, at which time said state machine moves into the
normal state corresponding to the second rotor position.
10. The positioning system of claim 9, further comprising:
a power supply having first and second terminals and connected to said
transistor switch pairs for supplying current to said coils through said
transistor switches;
wherein the preselected value for said first and second sense transistors
is approximately equal to a voltage at one of the power supply terminals.
11. A method for controlling a stepper motor having two coils, comprising
the steps of:
providing current to the coils through transistors connected thereto,
wherein two transistors are connected to a first end of a first coil, two
transistors are connected to a first end of a second coil, and two
transistors are connected to a common node to which a second end of both
coils are connected;
controlling the transistors to be on or off, selected combinations of
switch states defining steady states, and other selected combinations of
switch states defining transition states;
receiving a control signal; and
in response to the control signal, controlling the transistor switches to
move from a present steady state to a selected transition state, followed
by a move from the selected transition state to a next steady state,
wherein the move from the selected transition state occurs when currents
flowing through the coils reach preselected values.
12. The method of claim 11, wherein the control signal indicates which next
steady state is desired.
13. The method of claim 10, further comprising the steps of:
sensing the currents flowing through the coils; and
controlling the transistor switches to the next steady state when the
sensed currents reach preselected values.
14. The method of claim 11, further comprising the step of:
when in a steady state, controlling the current through the coils by
limiting it to a preselected maximum value.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to controls for electric motors,
and more specifically to a control system for controlling operation of a
stepper motor.
2. Description of the Prior Art
The construction, control, and use of stepper motors is well known to those
skilled in the art. Because of their design, stepper motors are able to
accurately position the rotor at multiple known locations. Because of this
capability, stepper motors have many uses. For example, they are used to
position the platen and print head in electronic typewriters and printers.
Controllers for stepper motors are used to energize the field coil windings
in selected sequences to change the position of the armature. Various
switching sequences of the different field coil windings are used to give
full step and half step positioning of the armature. Stepper motors can
also hold the armature in a fixed position. This is typically referred to
as detent mode.
Stepper motors typically have 2 independent field coils, but other numbers
may be used. For example, it is known to construct stepper motors having 5
field coils. The number of field coils, and the number of armature pole
pairs used, determine the angular separation between each full or half
step position. For example, a motor having 50 armature pole pairs and 2
field windings may be positioned at 200 different settings in the full
step mode. This corresponds to an angular separation of 1.8.degree..
Controlling this same motor in half step mode gives 400 positions, each
0.9.degree. apart.
A H-bridge is provided for each field coil. Switching of transistors in the
bridge is used to cause current to flow in either direction through the
coil as desired. The transistors can also be turned off so that no current
flows from the power supply through the coil. Controlling the switching of
the transistors for the two coils steps the motor in a forward or reverse
direction as desired.
Each bridge has four power transistors, so that eight are required for a
typical two coil motor controller. These power transistors may be bipolar
junction transistors or field effect devices. Since power transistors must
conduct large driving currents, on the order of one or two amps for a
typical medium-sized stepper motor, they must be physically large. As
known in the art, large devices result in a low level of integration, so
that control functions must be designed as separate chips.
It is desirable to increase integration levels for integrated circuits
generally. It would be desirable to increase integration levels for
stepper motor control circuitry. It would be further desirable to provide
a control system for a stepper motor which utilized a lesser number of
power transistors, allowing increased levels of integration.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a system and
method for controlling stepper motors.
It is another object of the present invention to provide such a system and
method which uses less than eight power transistors while still
accomplishing the same driving functions as are presently available.
It is a further object of the present invention to provide such a system
and method which is compatible with standard stepper motors, so that the
motor can be driven using the same control signals as are presently
provided for stepper motor control.
Therefore, according to the present invention, a control system for
controlling a stepper motor uses six power transistors in a single,
interconnected configuration with two stepper motor field coils. Two
transistors are common to both field coils of the stepper motor with two
stepper motor field coils. Control circuitry for switching the six
transistors allows full and half step motor positioning to be performed.
Transition phases are used between steady state phases to provide accurate
motor operation. The transition phases are controlled entirely within the
control circuitry without a requirement for input from outside the
controller.
BRIEF DESCRIPTION OF THE DRAWINGS
The novel features believed characteristic of the invention are set forth
in the appended claims. The invention itself however, as well as a
preferred mode of use, and further objects and advantages thereof, will
best be understood by reference to the following detailed description of
an illustrative embodiment when read in conjunction with the accompanying
drawings, wherein:
FIG. 1 illustrates diagrammatically and in tabular form current flows
required to operate a two coil stepper motor;
FIG. 2 is a schematic diagram of switching circuitry used to drive a
stepper motor according to the present invention;
FIG. 3 is a block diagram illustrating operation of a two coil stepper
motor according to the present invention;
FIGS. 4-11 illustrate current flows through the switching circuitry of FIG.
3, for eight steady state and transitional phases;
FIG. 12 is a table indicating transitional phases to be used in
transitioning between pairs of steady state phases;
FIG. 13 is a block diagram of control circuitry for driving the switching
circuitry of FIG. 3; and
FIG. 14 illustrates a circuit through stepper motor field coils.
DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1(a) illustrates diagrammatically a stepper motor generally referred
to with reference number 10. A rotor 12 is constructed as known in the
art, and can be positioned precisely to drive external machinery (not
shown). Field coils 14, 16 are energized in preselected patterns to
position the rotor 12. The direction of current flow through the coils 14,
16 determines the direction of magnetization of armature pairs 18, 20,
respectively. The motor 10 is shown diagrammatically as having only two
armature pole pairs, but an actual motor typically has many more as known
in the art.
FIG. 1(b) illustrates the eight rotor positions for a motor having two
armature pole pairs. Each position is 45.degree. apart, assuming a
half-step mode of operation. In full step mode, only the odd numbered
positions are used, and they are 90.degree. apart.
As is known in the art, if a motor having a greater number of armature pole
pairs is used, each position corresponds to a smaller angular
displacement. For example, a stepper motor having four armature pole pairs
has an angular displacement of 22.5.degree. for each half-step. There are
still only eight positions which can be defined by currents flowing
through the field coils. The 16 positions defined by a two armature pole
pair motor can be visualized as two consecutive sets of positions 0-7,
with each 0-7 set occupying 180.degree. of rotation. Thus stepping from
position 7 to position 0 moves the rotor into the next set.
FIG. 1(c) describes current flow through each of the coils, 14, 16 to
define each of the eight positions shown in FIG. 1(b). For example, in
position 1 corresponds to a positive current flow through both coils, and
position 7 corresponds to a positive flow through coil 14 and a negative
flow through coil 16. In position 2, coil 16 has current flow in a
positive direction, and no current flows through coil 14.
The rotor position shown in FIG. 1(a) corresponds to current flows through
coils 14, 16 which causes armatures 18, 20 to have north magnetic poles
below and to the right of the rotor 12. Assuming that positive current
flow is defined as going into the leads of the coils 14, 16 in the
direction of the arrows shown in FIG. 1(a), the rotor in FIG. 1(a) is in
position 7 as defined in the table of FIG. 1(c). For the rotor to be in
the position shown, the north poles of the armatures are located below and
to the right of the rotor 12, caused by positive current flow into coil 14
and negative current flow through coil 16.
FIG. 2 shows preferred power switching circuitry for controlling the
stepper motor according to the present invention. Coils 14 and 16 are
connected at common node 22. For both coils 14, 16, positive current flow
is defined as being through the respective coils toward the common node
22.
Three identical legs of power transistors are provided. The first leg
consists of transistors 24 and 26. The second leg consists of transistors
28, 30. The third leg consists of transistors 32, 34. All of the
transistors 24-34 are identical.
Each transistor 24-34 contains a parasitic diode 36 which is formed
naturally as part of the process of fabricating field effect transistors.
Transistors 24, 28, 32 are tied in common to a power supply Vs, and
transistors 26, 30, 34 are tied in common to node 38. Sense resistor 40
(Rs) is preferably a low resistance, precision resistor used to sense
current flow in the system. Resistor 40 functions in a similar manner to
sense resistors used each H-bridge for stepper motor control systems
according to prior art designs.
FIG. 3 illustrates a single chip motor controller 42 which can be
constructed in accordance with the teachings herein, and connected to
motor 10. The two coils 14, 16 are connected at node 22. The only
connections required between the motor 10 and the controller 42 are the
two coil leads 44, 46, and a connection to the common node 22. If desired,
the common node 22 can be located on the motor controller chip 42 with
four coil connections being made to the coils 14, 16.
The controller 42 contains the six power transistors 24-34, as well as
control circuitry (described below) for switching each of the power
transistors on or off in order to drive the rotor 12 as desired. The
controller 42 requires very few inputs, with those necessary being shown
in FIG. 3. A power supply Vs and ground connections are required. A
voltage reference input 48 is supplied, and is used for comparison with
voltages measured across sense resistor 40 in order to determine levels of
current flow through the motor. A clock input 50 causes the rotor 12 to
commutate (step) from its current position to an adjacent position. Each
pulse input to the clock input 50 causes the rotor to step one position.
The direction of rotor commutation is defined by an input direction signal
52, and a mode input 54 is used to control whether such rotor commutation
is performed in full step or half-step mode.
If desired, a separate voltage supply and ground (not shown) can be
provided for the logic circuitry within the motor controller 42. This
tends to insulate the control circuitry from supply bounce effects caused
by switching the power transistors.
FIGS. 4-11 show the current flows through the power switching transistors
24-34 for the different rotor positions shown in FIG. 1(c). Transitional
phases from the next lower numbered position are also shown in each
figure. Subfigure (a) of each of FIGS. 4-11 shows the transition phase and
Subfigure (b) shows current flows for the actual position (steady state
phase). For example, FIG. 4(b) shows the current flows for position 0, and
FIG. 4(a) shows the current flows for a transitional phase from position 7
to position 0. FIG. 5(a) shows current flow for a transitional phase
between position 0 and position 1, while FIG. 5(b) shows the currents for
the steady state phase of position 1.
Three different types of current flow will be referenced in the following
description. The first type is a steady state, or increasing to steady
state, current flow caused by application of a supply voltage across one
or both of the field coils 14, 16. This is shown in FIGS. 4-11 as an
unbroken line.
The other two types of current are caused by the collapse of the magnetic
field in the inductive coils when the power source is removed. Both of
these currents are shown in FIGS. 4-11 as dotted lines. As is known in the
art, these currents decay exponentially as a function of the voltage drop
across the coils. A higher voltage drop across the coil implies a higher
current flow and thus a faster decay. These two currents will be referred
to herein as a slow recirculation current, and a fast decay current. These
terms relate to the rate at which the current through the inductor decays.
The difference in these two currents can be described in connection with
FIG. 4(a). For purposes of the following descriptions, an exemplary
preferred embodiment assumes that the source Vs is a 48 volt source. In
addition, the assumption is made that the voltage drop across any of the
transistors 24-34 is approximately 0.3 to 0.5 volts when such transistors
are on. A similar voltage drop occurs across the parasitic diodes of such
transistors when current flows through them.
Prior to the transition phase shown in FIG. 4(a), the rotor is assumed to
be in position 7. As shown in FIG. 1(c), and diagrammed in FIG. 11(b),
current is flowing from left to right through both of the field coils 14
and 16. To enter transition phase 0 (FIG. 4(a)), transistors 24-34 are
switched into the positions shown. Transistors 26 and 30 are on, while the
remaining transistors are turned off. Both field coils 14, 16 previously
had current flowing from left to right, causing a magnetic field to be
generated in each coil. Once the power supply is removed, which occurs
when all of the upper transistors 24, 28, 32 are turned off, collapse of
these magnetic fields causes current to continue flowing in the same
direction.
As known by Kirchoff's voltage law, the sum of the voltages around the
lower left loop must be 0 volts. Since the on voltages for transistors 26
and 30 is approximately 0.3 to 0.5 volts, the voltage drop across field
coil 14 will be approximately 0.8 volts. At this low voltage, the magnetic
field in the coil 14 decays relatively slowly. Current indicated by flow
line 56 is relatively low, and is referred to as a slow recirculation
current.
Current indicated by flow line 58 flows through a loop consisting of
resistor 40, transistor 30, field coil 16, transistor 32, and the power
supply. Since the power supply is fixed at 48 volts, the voltage drop
across the remaining elements must also be 48 volts. As described above,
the voltage drop across transistors 32 and 32 is approximately 0.4 volts,
and the drop across resistor 40 is low, typically a few tenths of a volt.
Thus, the drop across field coil 16 is in the neighborhood of
approximately 47 volts, causing a relatively high loop current to flow.
Since the voltage drop across field coil 16 is roughly 47 volts and the
drop across field coil 14 is less than one volt, the magnetic field of
coil 16 collapses approximately 50 times faster than that of coil 14. For
practical purposes, the current through field coil 14 remains relatively
constant, while that through coil 16 diminishes quickly.
The arrowheads on current flow lines 56, 58 indicate the direction of
current flow. Thus, slow recirculation current 56 through field coil 14
circulates in a clockwise direction through the lower left quadrant of the
power switching circuitry. Fast decay current 58 flows generally upwardly
through the switching circuitry, and returns through the power supply.
As described above, the current flows of FIG. 4(a) are used merely as a
transition phase. Once the fast discharge current 58 has decayed to 0
volts, the control circuitry switches the power transistors 24-34 to the
positions shown in FIG. 4(b). This corresponds to position 0 of the rotor.
In position 0, current flows through coil 14 in a positive direction
(toward node 22), while no current flows through field coil 16. Current
line 60 indicates such current flow.
As known in the art, a high voltage Vs is applied to a stepper motor
designed for lower voltages in order to improve response time. Typically,
a Vs of 48 volts is applied to a 5 volt stepper motor. If the motor is
designed to carry one ampere of current, field coils 14 and 16 have a
nominal resistance of 5 ohms. If 48 volts were simply impressed across
field coil 14, a current of 10 amperes would result, burning out the
motor. It is therefore necessary to provide current limiting so that the
current through coil 14 is limited to approximately one ampere.
As is known in the art, such limiting can be performed by using a
"chopping" circuit. This control circuit senses the current through coil
14 by measuring the voltage drop across sense resistor 40. If resistor 40
has a value of one ohm, a one volt drop across it indicates a current flow
of one ampere. When a one volt drop is detected, transistor 30 is turned
off for a short period of time. This allows the current through field coil
14 to decay slightly, at which time transistor 30 is again turned on.
Thus, the current 60 is actually an oscillating current having a DC value
of approximately one ampere, and a small amplitude.
The term "chop" applied to transistors 28 and 30 indicates that they are
alternately turned on and off to perform this current limiting function.
Transistors 28 and 30 are turned off and on in a complementary fashion, so
that transistor 28 is on whenever transistor 30 is off. When transistor 30
is on, current 60 flows as indicated in FIG. 4(b). When transistors 28 and
30 change state, a slow recirculation current 62 is induced in the upper
left quadrant of FIG. 4(b), circulating in a counterclockwise direction.
Since both transistors 24 and 28 are on at this time, only a small
(approximately 0.8 volts) voltage drop occurs across field coil 14, so
that its discharge rate is relatively slow. Transistors 28, 30 then change
state again, allowing current 60 to again flow as shown. The power
transistors remain in the setting shown in FIG. 4(b) until it is desired
to move the rotor from position 0 to position 1.
FIG. 5 illustrates the position 1 transition phase (5(a)) and the position
1 steady state phase (FIG. 5(b)). To move from position 0 to position 1,
generally, it is necessary to hold the current already flowing through
field coil 14, and simultaneously generate a current flowing through field
coil 16 in the positive direction. Thus, transistors 24-34 are switched to
the positions shown in FIG. 5(a). This causes a slow recirculation current
64 to flow through field coil X, while current 66 builds up through field
coil 16. Current 66 builds up to the rated value of the motor fairly
quickly because of the high voltage Vs as described above. Once current 66
has reached the full rated value, the control circuitry causes power
transistors 24-34 to switch to the position shown in FIG. 5(b).
As shown in FIG. 5(b), currents flow from the power supply through
transistors 24, 32, through field coils 14, 16 toward node 22, and to
ground through transistor 30. Currents 68 and 70 are equal in magnitude.
Total current through both coils 14, 16 is sensed across sense resistor
40, and this signal is used to provide chopping on transistors 28, 30.
When transistor 30 is turned off, transistor 28 is turned on and
recirculation currents 72 and 74 are set up in the upper left and upper
right quadrants, respectively. The voltage drop across each of the field
coils 14, 16 is less than approximately 1 volt since only two transistor
voltage drops are found in each loop.
Since two coil currents are flowing through resistor 40 at the same time,
the chopping circuit should not be set to trigger on the same value as it
is set for when only one coil current is flowing. Assuming that a current
of one ampere flows through each coil in the steady state position, the
chopping circuit could be set to trigger on a two ampere current flow
through sense resistor 40. However, it is known in the art that rotor
movement is smoother if the vector sum of the fields in each rotor
position is the same. This is caused in position 1 by having each of the
currents 68, 70 have a value of approximately 0.7 amperes, which is
SQRT(2)/2. Thus, in position 1 the chopping circuitry is preferably set to
trigger when the current sense resistor 40 is equal to approximately 1.4
amperes.
FIG. 6(a) shows the transition phase from position 1 to position 2. Field
coil 14 discharges through fast decay current 76, while the current
through field coil 16 is maintained by slow recirculation current 78. When
the current through field coil 14 decays to 0, the control circuitry
switches the power transistors 24-34 into the positions shown in FIG.
6(b).
FIG. 6(b) is the position 2 steady state. In this state, no current flows
through field coil 14, while the rated current flows through field coil
16. Transistors 28 and 30 are chopped in the same manner as occurred in
position 0 described in FIG. 4(b). When transistor 30 is on, the current
through field coil 16 builds up to the rated value. When the voltage
across sense resistor 40 indicates that the rated value has been reached,
transistor 28 is turned on while transistor 30 is turned off. This causes
recirculation current 82 to flow through the upper right quadrant of FIG.
6(b) until the states of transistors 28 and 30 are again changed.
FIG. 7 shows the current flows for position 3. FIG. 7(a) shows the position
3 transition state, in which slow recirculation current 84 maintains the
current flow through field coil 16. It is necessary to build up a current,
in the negative direction, through field coil 14. This is accomplished by
causing current to flow through transistor 28, field coil 14, and
transistor 26. Once current 86 has built up to the rated value, transition
phase 3 ends and the position 3 steady state phase, shown in FIG. 7(b)
begins.
Current 88 flows through both of the field coils 14, 16 in series. When
current 88 reaches the rated value, transistor 26 is turned off, and
transistors 24 and 28 are both turned on. Note that transistor 28 is off
while transistor 26 is on. This allows slow recirculation currents 90, 92
to maintain current flow through both of the field coils 14, 16
respectively.
As described above, it is preferable to have a current flow of only 0.7
amps (assuming a 1 ampere rated current) when current is flowing through
both coils. Thus, the chopping circuitry is set to trigger when current 88
reaches a value of 0.7 amperes.
The transition phase to position 4 is shown in FIG. 8(a). In this
transition, it is necessary to maintain the negatively flowing current
through field coil 14, while allowing the current through coil 16 to
discharge to 0. Therefore, the transistors are switched as shown, and slow
recirculation current 94 maintains the current flow through field coil 14.
Fast decay current 96 quickly decays to 0.
When current 96 decays to 0, the position 4 steady state is entered as
shown in FIG. 8(b). In this position, a negatively directed current 98
flows through field coil 14. While current 98 is flowing, transistor 28 is
on and transistor 30 is off. When current 98 reaches the rated value,
transistors 28 and 30 are switched so that recirculation current 100
circulates in a counterclockwise direction through the lower left
quadrant.
In position 5, negatively flowing current (directed away from node 22)
flows through both field coils 14, 16. Therefore, the switching state
shown in FIG. 9(a) is used to define the position 5 transition phase. Slow
recirculation current 102 is allowed to flow through field coil 14, while
current 104 builds up to the rated value through field coil 16. When
current 104 reaches the rated value, the position 5 steady state phase is
entered as shown in FIG. 9(b).
Currents 106, 108 flow in the negative directions through field coils 14,
16, respectively. As described above, the chopping circuitry preferably is
triggered when the sum of these currents is approximately 1.4 times their
rated value for each coil. Slow recirculation currents 110, 112 maintain
the current through the field coils 14, 16 during chopping.
FIG. 10(a) illustrates the transition phase from position 5 to position 6.
It is necessary to discharge field coil 14 with fast decay current 114,
while maintaining the current through field coil 16 through recirculation
current 116. When current 114 decays to 0, the position 6 steady state
phase is entered as shown in FIG. 10(b).
In steady state position 6, current 118 is maintained at approximately the
rated value. During chopping, slow recirculation current 120 maintains the
current through field coil 16. No current flows through field coil 14.
FIG. 11(a) shows the position 7 transition phase from position 6. The
current through field coil 16 is maintained with slow recirculation
current 122, while the current 124 through field coil 14 is increased to
the rated value. when current 124 reaches the rated value, the position 7
steady state phase is entered as shown in FIG. 11(b).
Current 126 flows through both field coils 14, 16, and preferably is
maintained at a value of 0.7 amperes as described above. During chopping
recirculation currents 128, 130 maintain the currents on coils 14, 16
respectively. Three transistors, 28, 32, and 34 are involved in the
chopping process in the same manner as described in connection with FIG.
7(b).
Each of the steady state positions described above was described with a
corresponding transition phase. This transition phase is the one which is
used when moving from a lower numbered steady state phase to the higher
phase. The position 0 transition phase is used when moving from position 7
to position 0.
It will be appreciated by those skilled in the art that the purpose of the
transition phase is to adjust the coil current in preparation for the next
steady state phase. This ensures that, when currents flow through both
coils 14, 16, the currents are equal. If the next position was entered
directly, without the transition phase, the currents to the two coils
would be different. Since the chopping circuitry senses only the sum of
the currents through the two coils, it is not capable of independently
determining, and controlling, the current through the two coils in order
to equalize them.
When the rotor is commutated in the opposite direction, from a higher
number position to a lower position, different transition phases must be
used. For example, when transitioning from position 4 to position 3, the
position 3 transition phase is not used. Instead the position 1 transition
phase is used to maintain the current on field coil 14 while building up
the current through field coil 16.
FIG. 12 is a table indicating the proper transition phases to be used when
moving a half-step between an initial position and an adjacent final
position in either direction. Consistent with the earlier description,
moving from a lower numbered initial position to a higher numbered final
position utilizes the transition phase associated with the final position.
Different transition phases are used for moving in the opposite direction,
as shown in the table.
For example, when moving from position 2 to position 3, the transition
phase associated with position 3 is used. It is necessary to maintain the
current through coil 16 while establishing a current through coil 14.
However, when moving from position 3 to position 2, the table of FIG. 12
indicates that the transition phase associated with position 6 should be
used. This transition phase is appropriate because it is necessary to
discharge the field coil 14 while maintaining the current on field coil
16. FIG. 12 provides a complete table of transition phases used when
moving between any pair of steady state positions in half-step mode.
If the motor is operated on full step mode, only rotor positions 1, 3, 5,
and 7 are used. A similar transition phase is utilized between each steady
state position as in the half-step mode. Normally, half-step operation is
more common than the use of the full step mode.
FIG. 13 shows a block diagram of control circuitry 132 used for driving
power switching transistors 24-34. A state machine 134, constructed
according to principles well known in the art, causes the appropriate
transitions between positions as described above. As input, the state
machine 134 requires a clock signal 136 and a direction signal 138. Each
time a pulse occurs on the clock input 136, a transition is made from the
current position to one of the two adjacent positions. The direction input
138 controls the direction of such transition. The state machine
implements the table of FIG. 12 to determine which transition phase to
enter, and a change is made to the target steady state position when a
signal from the decay current sense circuitry 140 indicates that the
appropriate coil current has reached its desired value.
Signal routing circuitry 142 converts the outputs 144 of the state machine
134 into the six control gate signals required to be applied to the
transistors 24-34. The outputs 144 are determined using the switch states
as described in FIGS. 4-11, and can easily be implemented by a person
skilled in the art.
Monostable circuit 146 is used to control the chopping functions, and
provides an output 148 to be connected to the signal routing circuitry
142. The transistors which are to be controlled by the chopping circuitry
output 148 are defined by the state machine output signals 144. The
monostable circuit 146 senses the current through sense resistor 40 to
determine when the chopping function should occur. Miscellaneous control
signals 150 may also be provided to the monostable circuit to program it
to function as desired.
A preferred design for monostable circuit 146 is described in currently
co-pending U.S. patent application Ser. No. 07/414,699 titled Programmable
Stepper Motor Controller, assigned to the Assignee hereof now U.S. Pat.
No. 5,032,7 . Other circuitry for performing the same function, as known
in the art, may be used instead.
The decay current sense circuitry 140 may be implemented in several
different ways. All of the required sensing can be performed across the
sense resistor 40 if desired. Performing this function requires that the
amplifier for sensing the voltage across the resistor 40 be able to sense
the voltage drop in either direction. For example in the transition phase
for position 0, the required sensing is to determine when current flowing
upward through resistor 40 reaches 0.
Sensing of voltage drop in either direction across resistor 40, with one
end tied to ground, requires that some portion of the circuitry carry a
negative voltage. This may not be consistent with the design of a CMOS
control circuit, and such bidirectional sensing may not be available. If
this is the case, it is necessary to provide an alternative arrangement
for sensing when a fast decay current has reached 0.
One such current sensing circuit is shown in FIG. 14. It connects to a node
160 between upper and lower transistors 162, 164. The sensing circuit is
used on both the A and C legs of the motor drive circuitry, so transistors
162 and 164 correspond respectively to either transistors 24 and 26, or to
transistors 32 and 34. Parasitic diodes 36 operate as described
previously.
When a motor coil 14 or 16 is discharging from ground, as shown in FIGS.
4(a), 6(a) 8(a) and 10(a), the circuit of FIG. 14 is used to detect the
event that the coil has fully discharged. When current is flowing through
sense resistor 40 toward node 38, the voltage drop across the resistor 40
and the lower parasitic diode 36 causes the voltage at node 160 to be
below ground potential. This turns diode 166 on. When diode 166 is on, the
base of transistor 168 is at or below ground, and the base-emitter
junction of transistor 168 is reverse biased, turning transistor 168 off.
The transistor 168 remains off until the voltage at node 160 rises to
approximately ground potential, which occurs when the motor coil is nearly
fully discharged. When the voltage at node 160 rises, diode 166 is reverse
biased and transistor 168 turns on. Resistor values for resistors 170 and
172 are chosen so that transistor 168 is on when diode 166 is reverse
biased.
When transistor 168 is on, the voltage Vo is low. This indicates that the
coil connected to node 160 has not fully discharged. When the transistor
168 is off Vo is high, indicating that the transition phase discharged is
not yet complete. The value of Vo is considered only during the transition
phases which require a coil to discharge as shown in FIGS. 4(a), 6(a),
8(a), and 10(a). Vo transitions low to indicate the end of the transition
phase. The value of Vo is ignored at other times. The control system
described herein provides a technique for driving a stepper motor which
uses a total of only six power switching transistors. The control function
for six transistor switching circuit is more complicated than typically
found in prior art eight power transistor devices, but can still be
implemented in a straightforward manner. The extra control circuitry
requires much less space on an integrated circuit chip than the two power
transistors which are replaced. Thus, even with the more complex control
function, the described system can be more easily integrated onto a single
chip than can prior art systems. The functioning of the stepper motor is
identical to prior art techniques, as far as the user is concerned, since
the newly introduced transition phases are handled completely internally
to the control circuitry. Thus, it is not necessary for a designer to
learn new techniques for controlling stepper motors to make use of the
described invention.
While the invention has been particularly shown and described with
reference to a preferred embodiment, it will be understood by those
skilled in the art that various changes in form and detail may be made
therein without departing from the spirit and scope of the invention.
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